[leetcode] 438. Find All Anagrams in a String 解題報告

題目鏈接：https://leetcode.com/problems/find-all-anagrams-in-a-string/

Given a string s and a non-empty string p, find all the start indices of p's anagrams in s.

Strings consists of lowercase English letters only and the length of both strings s and p will not be larger than 20,100.

The order of output does not matter.

Example 1:

Input:
s: "cbaebabacd" p: "abc"

Output:
[0, 6]

Explanation:
The substring with start index = 0 is "cba", which is an anagram of "abc".
The substring with start index = 6 is "bac", which is an anagram of "abc".

Example 2:

Input:
s: "abab" p: "ab"

Output:
[0, 1, 2]

Explanation:
The substring with start index = 0 is "ab", which is an anagram of "ab".
The substring with start index = 1 is "ba", which is an anagram of "ab".
The substring with start index = 2 is "ab", which is an anagram of "ab"

思路：直接的方式是暴力用hash，這樣時間複雜度O(m*n)．可以用hash+slide window來優化到O(n)．

代碼如下：

class Solution {
public:
    vector<int> findAnagrams(string s, string p) {
        vector<int> hash1(256), hash2(256);
        for(auto ch: p) hash1[ch]++;
        int lenp = p.size(), lens = s.size();
        vector<int> ans;
        for(int i = 0; i < lens; i++)
        {
            hash2[s[i]]++;
            if(i >= lenp) hash2[s[i-lenp]]--;
            if(hash1 == hash2) ans.push_back(i-lenp+1);
        }
        return ans;
    }
};